The fabricating and processing of semiconductor wafers to produce individual integrated circuits (IC's) is well known in the art. In one widely used manufacturing method a wafer (e.g., an eight inch diameter, silicon wafer) is chemically and photographically processed through a number of steps to produce a multitude of very closely spaced and precisely detailed IC's on the wafer. As part of its processing, a wafer may be exposed within a reactor to a highly active plasma of special gas or gases in order to etch, by means of reactive ions of the gases, very fine details (lines, zones, etc.) into a top surface of a wafer being processed. The wafer is subsequently cut into individual IC's. This general technology is well known in the art and need not be described further.
A typical plasma etching apparatus comprises a reactor in which there is a chamber through which reactive gas or gases are flowed. Within the chamber the gases are ionized into a plasma, typically by radio frequency energy. The highly reactive ions of the plasma gas are able to react with material, such as a polymer mask on a surface of a semiconductor wafer being processed into IC's. Prior to etching, the wafer is placed in the chamber and held in proper position by a chuck or holder which exposes a top surface of the wafer to the plasma gas. There are several types of chucks (also sometimes called susceptors) known in the art. The chuck provides an isothermal surface and serves as a heat sink for the wafer. In one type, a semiconductor wafer is held in place for etching by mechanical clamping means. In another type of chuck, a semiconductor wafer is held in place by electrostatic force generated by an electric field between the chuck and wafer. The present invention is applicable to both types of chucks.
During etching in a typical plasma etching operation, the reactive ions of the plasma gas chemically react with portions of material on a face of the semiconductor wafer. Some processes are exothermic and cause some degree of heating of the wafer, but most of the heating is caused by the plasma. The chemical reaction between gas (ions and radicals) and wafer material, on the other hand, is accelerated to some degree by the temperature rise of the wafer. Local wafer temperature and rate of chemical reaction at each microscopic point on the wafer are related to an extent that harmful unevenness in etching of material over a face of the wafer can easily result if the temperature of the wafer across its area varies too much. In most cases it is highly desirable that etching be uniform to a nearly perfect degree since otherwise the IC's being fabricated will have electronic characteristics which deviate more than is desirable. Furthermore, with each new increase in the size of wafer diameter, going from four inch to six, to eight and in the near future to twelve inch diameter, the problem of insuring uniformity of each batch of IC's from larger and larger wafers becomes more difficult.
The problem of temperature rise of a wafer during reactive ion etching (RIE) is well known, and various attempts in the past to control the temperature of a wafer during etching have been tried. One previous way to control wafer temperature during RIE has been to admit coolant gas (such as helium) at a single pressure within a single thin space between the bottom of the wafer and the top of the chuck which holds the wafer. However, past arrangements of this sort have not been entirely effective in adequately controlling rises and variations in wafer temperature. This is particularly so with larger diameter wafers. It is desirable therefore to provide apparatus and method for improved control of temperature of semiconductor wafers during RIE and similar processes.